This invention deals with printer mechanisms using non-impact type printers and more specifically with printer mechanisms using relatively wide thermal printheads having a plurality of mesas adapted for printing a plurality of characters as the same time. The relatively wide printhead must be pressed against a printing medium which is driven by a platen. To achieve uniform print density and clarity, it is desirable that the pressure across the entire printhead be substantially the same.
In the prior art, non-impact type printhead have moved across the print medium to form characters in a step-by-step fashion. The printhead required for this type printer is generally quite small and therefore the application of even pressure is a relatively simple matter. However, as the printhead is made wider, slight variations in the platen and in the print medium may cause the printhead to fully contact one portion of the medium and bearly touch other portions. To overcome this problem, prior art techniques have envolved the very carefully machining of the cylinder-shaped platten to ensure surface uniformity and the use of very tight construction tolerances to assure that the plane in which the printheads occur is tangential to the platen.
Also in the prior art, as exemplified in U.S. patent application Ser. No. 680,835, relatively complex printhead alignment mechanisms have been used to assure that relatively wide printheads may be urged against a platen with substantially even pressure thereby yielding highly satisfactory print density and clarity. The print alignment mechanism of U.S. patent application Ser. No. 680,835 is often embodied in printing type calculators. With the advent of large scale integration techniques which have reduced the number of chips in typical calculators to as few as a single chip, the cost of calculators in general to the consuming public has dramatically dropped. While the printhead alignment mechanism of U.S. patent application Ser. No. 680,835 is a satisfactory design, the relative complexity thereof results in a comparatively expensive printer unit for a desk model printing calculator for instance.
It was therefore an object of this invention that an improved printer mechanism be provided for printing calculators.
It was another object of this invention that a relatively wide printhead be disposed with substantially even pressure against a platen.
It was yet another object of this invention that the resulting printer unit utilize a small number of parts which may be easily assembled and inexpensively procured.
The foregoing objects are achieved as is now described. A platen is rotatably mounted above a base. A medium upon which printing is to occur, preferably thermally sensitive paper, is disposed adjacent to the platten. A relatively wide thermal printhead is disposed against the medium by means of a holder. The holder is mounted on a stand which is connected to the base; the mount is preferably provided by a ball and socket arrangement which permits the holder to pivot about first and second orthogonal axes with respect to the platen. The holder and stand also include a projection and channel arrangement to inhibit the holder from pivoting about a third axis, the third axis being orthogonal to both the first and second axes. The holder includes a lead frame assembly, the ends of which are formed to provide electrical contacts in the holder. The contacts in the holder are disposed in contacting relationship to contacts on the thermal printhead when the thermal printhead is retained by the holder. The lead frame is preferably disposed in two major planes, one of which is preferably essentially parallel to a major plane of the holder. The other end of the lead frame is preferably connected to a printed circuit board on which the other electronic components associated with the printer are preferably mounted. A spring is preferably connected to the holder and to the base for urging the holder toward the platen.